It is known, e.g. from U.S. Pat. No. 3,706,116, to provide the plates of such a tripartite mold with a coupling including a control bar rigid with a stationary first outer plate, a locking bar rigid with a movable second outer plate, and a detent on an intermediate plate sandwiched between the two outer plates which latches the locking bar to the intermediate plate in the mold-closed position. When the two outer plates are driven apart to open the mold, the intermediate plate is entrained by the second outer plate in a direction away from the first outer plate so as to open a first interplate gap. After that gap has reached a certain width, the control bar rigid with the first outer plate coacts with the detent on the intermediate plate to release it from the locking bar while the intermediate plate is stopped or indexed in the position last reached. The second outer plate, continuing its displacement away from the first one, then separates from the intermediate plate to open a second interplate gap. In many instances, the freshly molded workpieces are ejected into this second gap while the first one is used to prepare for such ejection by the withdrawal of one or more cores from respective mold cavities.
In the known two-stage mold coupling, the two bars pass at different levels through a guide groove accommodating the detent in a block fastened to the intermediate plate. The detent is a pivoted latch in the shape of a segment of a circle having a control lug and a locking lug respectively coacting with the corresponding bars. In the mold-closed position, a spring urges the locking lug into a triangular recess of the locking bar under sufficient pressure to hold that bar engaged. When the outer plates have begun their separation, a generally gable-shaped camming formation on the control bar bears upon the control lug of the detent and represses same against its spring force with resulting disengagement of the locking lug from the other bar.
A drawback of the known coupling is the need for a considerable spring force to keep the intermediate plate locked to the second outer plate whereby a commensurate force is required for releasing these two plates from each other. On the other hand, the camming edge of the control bar makes only line contact with the control lug of the detent so as to give rise to considerable Hertzian stresses which in turn result in rapid wear of the parts involved.